This invention relates to optical memory and more specifically Optical Random Access Memory (O-RAM) and Optical Read Only Memory (O-ROM) for applications involving optical computing and optical fibre communications.
The need for data networks with the ability to move very large quantities of information over very long distances has resulted in optical networks replacing electrical networks for telecommunications and data services. The optical communication systems used to link the individual elements of a high-speed data network take advantage of wavelength division multiplexing or WDM. The propagation of multiple wavelengths of light within a single strand of optical fibre boosts the total bandwidth of the fibre but requires specialized equipment to carry out various functions such as separating the various wavelengths. Information travelling between the nodes of a SONET (synchronous optical network) or the Internet moves as pulses of light but are converted to electrical signals, and then converted back into light for being redirected. The conversion of light into electrical signals is carried out because, at present, redirecting optical signals purely within the optical domain is very difficult. In the Internet case, the difficulty stems from two main causes. First, to transmit the optical signal to the right destination location, the router must know what the correct destination is. To determine the right destination the router must read the information. To do this the router must convert the optical data to an electrical data packet for packet analysis. Second, once the router has determined a correct destination, the packet has already propagated beyond the router. While it has been proposed to send the header of the packet before the packet, specifically to address this problem, this option is not helpful in that if, for any reason, the router is not ready for the packet when it arrives, the communication link will not be successfully maintained. Currently, the packets are buffered by electric memories after the optical signals are converted to electrical ones. This limits the bit rate of the optical transmissions. Therefore an optical memory, designed for telecommunications purposes, to store optical signals such as an optical buffer is advantageous. When combining WDM or DWDM (Dense WDM) with the Internet, the wavelength channels within one fibre are used for transmitting the packets. In order for an optical memory to be suitable for this application it must be capable of receiving, storing and producing combinations of signals at individual wavelengths used by the network. In the all-optical wavelength routing WDM or DWDM network, it becomes essential to buffer the signal in the optical domain. This is because that the all-optical wavelength router requires a switching time to set up the route for the signal. Therefore, an optical memory and more specifically a random access optical memory is essential to the all-optical wavelength routing WDM network.
While advances in optical communications caused electrical long distance networks to become obsolete, the same is not true of computers. There are significant technological issues preventing the deployment of optical computers, one of which is that no random access optical memories are presently available. One of the advantages of optical signals over electrical signals is that they do not produce electric or magnetic fields. Also, unlike electrical signals, they are very insensitive to external electric and magnetic fields. This insensitivity is an advantage if the optical signal is being transmitted a long distance but it makes storing and guiding optical signals more difficult. As optical computers hold tremendous promise, many researchers have tried to develop optical memories.
As used herein, the terms xe2x80x9cstoringxe2x80x9d and xe2x80x9cretrievingxe2x80x9d optical signals occur very quickly and the storage of the signal itself occurs in the optical domain. This alleviates many of the problems with electro-optical solutions available today. Currently, storing and retrieving an optical signal involves an optical to electrical to optical (or OEO) conversion. As shown in FIG. 1, this conversion typically includes the following steps: A multi-wavelength optical signal enters a xe2x80x9creceiving modulexe2x80x9d. The optical signal is wavelength demultiplexed creating a set of optical signals, each of which has only one wavelength. Each optical signal in the set goes to a separate detector. The detectors convert each optical signal into a corresponding electrical signal. The electrical signal is digitised to quantise the data and the quantised data is then stored in a conventional electrical memory.
At some point the electrical system is instructed to send the optical signal by the following steps: The electrical signals are retrieved and provided to lasers for modulation thereof. The individual laser having the correct wavelength of the constituent optical signal is modulated to recreate a signal similar to the original optical signal. The different optical signals are then wavelength division multiplexed causing each of the signals to propagate within a single medium, typically an optical fibre.
This conversion is commonly used; however, there is a strong desire to eliminate it. The detectors and the lasers are very expensive and rare. The cost and availability of these components vary dramatically based on quality and performance. This conversion is needed to store the signal temporarily. Much of the cost is associated with converting the optical signal into an electrical signal and back again into the optical domain.
An optical memory more suitable for telecommunication purposes would be able to store the optical signal and later transmit the same signal without the need to convert it into an electrical signal and then back into an optical signal. Similarly, the signal used to trigger the writing or reading of the optical signal is preferably also optical. Finally, the signal is preferably stored in the optical domain, in other words, the stream of photons remains a stream of photons. There are optical memories that operate by storing optical information in the physical domain, such as a photograph. Unfortunately, these are not considered practicable solutions for telecommunications or other data transmission problems.
In a truly random access memory architecture any cell of the memory can be made to provide information at an output port thereof. A common approach to producing an optical memory is to create a waveguide that features a loop structure in which any optical signal on the loop propagates in a continuous circuit about the loop. This allows a stream of data to be stored and retrieved. Since the optical signal travels a long distance other problems arise, such as attenuation and dispersion. Technology has kept up with these problems to a large extent by inserting amplifiers within the loops to maintain the signal strength.
These problems are analogous to dispersion and degradation problems in long distance, high speed data networks. Ensuring that optical data do not suffer from signal degradation, specifically attenuation and signal dispersion, requires highly specialized optical components that are costly and can be difficult to obtain. As related technology advances and production techniques improve, the components needed to reduce signal degradation will likely become cheaper and more readily available. However, even with the best components, some signal degradation will be experienced if the optical signal is to be stored for an extended period of time using a loop design. The longer the signal is stored, the harder it will be to properly compensate for the reduction in signal quality.
In U.S. Pat. No. 5,740,117, a method of storing optical signals in a loop structure is described. The method allows for storing a fixed amount of information within an optical loop. In order to do this effectively, the system must keep track of the time when the data entered the memory, or it must be capable of recognizing a point or header within the optical signal. Since an optical signalxe2x80x94a stream of informationxe2x80x94is being stored, it is important that the signal not suffer degradation. Similarly, it is important that a control unit controlling the memory track an interval since the memory was loaded with the required information. In the event that the system accesses the memory at the wrong time, incorrect data from the stream is retrieved.
Ideally, a memory control unit would not have to know when the memory was last loaded. Also it is unclear how this style of optical memory could be used for multiple wavelengths simultaneously. It is known that different wavelengths of light propagate through waveguides at different speeds. The effect is known as wavelength dispersion. This would clearly be problematic in a system where the contents of the memory must be written at a very precise time. Further, the patent does not address multiwavelength optical storage.
The loop structure in the above example provides a buffer for storing the signal for a brief period of time. While this is useful in some applications it lacks flexibility. The option of storing optical information for an extended period of time is very desirable. Similarly, the ability to read the same information more than once, which is analogous to electronic random access memory, is beneficial. The loop architecture is shown in FIG. 2. A data signal enters the loop 20 and is routed by a coupler. The data stream stays in the loop 22 until the coupler allows it out.
In U.S. Pat. No. 5,999,283 in the name of Roberts and Whiteaway is disclosed an optical logic device with a Mach-Zender interferometer (MZI) featuring a pair of semiconductor optical amplifiers (SOA). This structure can be used to produce a variety of Boolean logic gates operating in the optical domain. Roberts builds upon this work in U.S. Pat. No. 5,999,284 by demonstrating that this structure can also be used to produce a latching optical memory. This patent shows how this architecture can be used to produce a variety of simple optical circuits. The electrical equivalents of these circuits are the basic building blocks of the Boolean electrical computer.
Unfortunately, the use of Boolean logic gates requires a single input signal to a single gate. As such, there is no extension of the logic to existing WDM optical networks.
The use of holograms to store data optically holds tremendous promise. The main advantage to this technique is that a small hologram is theoretically capable of holding very large quantities of data. However, a hologram is fundamentally different from an ideal optical memory for high-speed data networks because it stores the information in the physical domain, instead of storing the data in the optical domain. In operation, the hologram writes information into a medium and then illuminates the medium to retrieve the information. It would be advantageous to store the optical signal containing the light in more of an active system.
It would be very advantageous to design an optical memory supporting a known range of wavelengths, thereby making it suitable for WDM networks and related applications. Further, it is desirable to have the contents of the memory available immediately. In order to produce an optical computer it is preferable to have an optical memory whose control functions are all controlled by other optical signals and whose operation is very fast.